1. Field of the Invention
This invention relates to a light identifying system used on photoelectric switchs, measuring apparatus and others for identifying whether or not light from a light emitting part is received.
2. Description of the Prior Art
A prior art light identifying circuit will be described with reference to FIG. 5. In the drawing, reference numeral 11 denotes a light emitting part and 12 denotes a light receiving part, disposed opposite each other. When used as an optical switch, an object (moving member) 15 may be interposed between the light emitting part 11 and the light receiving part 12. Light from the light emitting part 11 is received by the light receiving part 12 or intercepted according to the position of object 15. Light emitting part 11 may be self-luminous or may be of a type excitedly an electrical signal. Light receiving part 12 is a conventional device effective for generating a light reception signal having an amplitude that varies according to the quantity of received light.
The aforementioned light reception signal is amplified by an amplifier 13. A combination of the light receiving part 12 and the amplifier 13 is called a photoelectric conversion part 14.
Reference numeral 16 denotes a comparator, wherein a threshold voltage set by a resistance 17, is applied to a minus (-) input. The threshold is compared with the light reception signal from the photoelectric conversion unit 14 which is impressed on the plus (+) input of comparater 16.
A condensing lens (not shown) is disposed on the front of the light emitting part 11 and the light receiving part 12.
In the aforementioned construction, the light emitting part 11 is supplied with a pulse signal as illustrated and emits a corresponding pulsating light. If the object 15 is not present at an intercepting position, the light reaches the light receiving part 12, where it is subjected to photoelectric conversion. The resulting light reception signal from the photelectric conversion unit 14 is compared with a threshold Vs in the comparator 16. If the object 15 is not present at an intercepting position, then a pulse signal, synchronized with an incoming signal to the light emitting part 11, is generated. On the other hand, if the object 15 is present at the intercepting position, a signal of the comparator 16 remains at a constant level "L".
Thus, whether or not the object 15 is present at a predetermined position is identified by the nature of the outgoing signal from the comparator 16.
The output of the aforementioned comparator 16 is either inverted or not according to whether or not a magnitude of the light reception signal is above or below the preset threshold Vs. This threshold is fixed in the prior art system. Therefore, when a quantity of light emitted by the light emitting part 11 increases or decreases over time, or the received light increases or decreases due to changing transmission conditions such as mist, oil film or the like, the light reception signal changes its relationship with the threshold. This may cause an operation error to occur. Consequently, the threshold must be readjusted to comply with the time-dependent and ambient changes.